Electro-optical modulators

ABSTRACT

An electro-optical modulator comprising an electro-optic crystal, means for projecting a beam of light through the crystal on to an optical aperture, and means for applying to the crystal a variable electric field which will be substantially non-uniform across the beam of light in any plane transverse to the path of the beam so as to cause a variable spreading of the beam and thereby modulate the amount of illumination passing through the optical aperture. The electric field may be applied to the crystal by means of two narrow linear electrodes on opposite sides of the crystal and parallel to the path of the beam. The optical aperture may be a translucent aperture in an opaque screen, or it may be one end of a fiber-optic light guide. An optical alignment system may be formed of one or two such electro-optic modulator crystals, an optical detector for producing an electrical signal representing the amount of illumination passing through the optical aperture and frequency selective means responsive to the said signal. Servo systems may be provided for adjusting the position of the aperture transversely so as to maximize the output of the frequency selective means.

llnite Dore tates atent ELECTRO-OPTICAL MODULATORS [72] Inventor: MarcusJohn Dore, Bournemouth,

England [73] Assignee: National Research Development Corporation,London, England [22] Filed: Aug. 13, 1969 [21] Appl.No.: 849,643

Electronic Design, Vol. 12, No. 15, July 20, 1964 Laser Beam Steered byElectro-Optic Deflector Cell. Journal of Scientific Inst. Nov. 19, 1964,V01. 41,

[4 1 Sept. 12, 1972 Methods of Modulating Light at Extreme Frequencies.by O. C. Jones.

Primary Examiner-Benjamin A. Borchelt Assistant ExaminerN. MoskowitzAttorney-Cushman, Darby & Cushman [57] ABSTRACT An electro-opticalmodulator comprising an electrooptic crystal, means for projecting abeam of light through the crystal on to an optical aperture, and meansfor applying to the crystal a variable electric field which will besubstantially non-uniform across the beam of light in any planetransverse to the path of the beam so as to cause a variable spreadingof the beam and thereby modulate the amount of illumination passingthrough the optical aperture. The electric field may be applied to thecrystal by means of two narrow linear electrodes on opposite sides ofthe crystal and parallel to the path of the beam. The optical aperturemay be a translucent aperture in an opaque screen, or it may be one endof a fiber-optic light guide.

An optical alignment system may be formed of one or two suchelectro-optic modulator crystals, an optical detector for producing anelectrical signal representing the amount of illumination passingthrough the optical aperture and frequency selective means respons iveto the said signal. Servo systems may be provided for adjusting theposition of the aperture transversely so as to maximize the output ofthe frequency selective means.

1 Claim, 3 Drawing Figures OPTICAL J2 DETECTOR 2f 2 2f I F2 Fl A2 Al 1ELECTRO-OPTICAL MODULATORS The present invention relates toelectro-optical modulators, which may for instance be used to modulate abeam of coherent light in an optical communications link.

Optical modulation may be achieved by using electro-optic effects invarious crystals, and it has been shown that the modulation signal powerrequired for a given application may be minimized by using a longslender crystal. However, none of the known modulator arrangements isreally convenient for use in a long slender crystal. Some of the knownarrangements make use of variable interference effects between ordinaryand extraordinary light waves in birefringent crystals, and are limitedby temperature-dependent effects and the tendency of the ordinary andextraordinary waves to become spatially separated. To reduce thesedifficulties, the crystals may be used in pairs in tandem and accuratelytemperature-controlled. An alternative possibility is to modulate thebeam by controlled deflections relative to a shaped aperture, but knownlightbeam deflection arrangements are also inconvenient for use with along slender crystal, requiring either a multiplicity of crystal prismsor a crystal with specially shaped hyperboloidal electrodes.

It is an object of the invention to provide a comparatively simpleelectro-optical modulator requiring only one crystal and convenientlyapplicable to a long and slender crystal.

According to the present invention there is provided an electro-opticalmodulator which includes an electrooptic crystal, means for projecting abeam of light through the crystal on to an optical aperture, and meansfor applying to the crystal a variable electric field which will besubstantially non-uniform across the light beam in any plane transverseto its direction of propagation, the orientation of the crystal and theelectric field being such that the velocity of propagation of the lightforming various parts of the beam will vary with the local variations inthe electric field, thereby tending to cause a spreading of the beamdependent on the strength of the applied electric field. The opticalaperture may be a transparent pinhole in an opaque screen, oralternatively it may be the end of a fiberoptic light guide. The crystalmay be of a material such as ammonium dihydrogen phosphate, potassiumdihydrogen phosphate or potassium di-deuterium phosphate. Thesematerials are commonly called ADP, KDP and KB? respectively. Othersuitable materials, such as lithium niobate, are now also known. Thedirection of propagation of the light beam through the crystal may be atright angles to the c-axis and at 45 to each of the a-axes of thecrystal, with the maximum electric field gradient aligned in thedirection of the caxis. The light beam is preferably polarized in aplane at 45 to the crystal c-axis. The crystal is preferably long in thedirection of propagation of the light and thin in the direction of theelectric field. The electric field may be applied through narrow linearelectrodes disposed parallel to the path of the beam of light onopposite sides of the crystal and as close as possible to the beam.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, of which FIG. 1 is aschematic perspective drawing of an electro-optical modulator includingan electro-optic crystal, and;

FIG. 2 is a transverse cross-section of an electrooptic crystal providedwith an alternative arrangement of electrodes and electrical connectionsthereto, and;

FIG. 3 is a schematic perspective drawing of an aperture alignmentapparatusincluding two electro-optical modulators.

FIG. 1 shows a laser L arranged to project a beam B of coherent lightlongitudinally through a long slender ADP crystal X1 and on to the endof a fiber-optic light guide G. Arrows marked 0, a and a indicate thedirections of the axes of the crystal X1; the c-axis is vertical in theembodiment shown. A modulation signal source M1 is electricallyconnected to a narrow linear electrode E1 formed on the top surface ofthe crystal X1 and to a similar electrode E2 formed on its undersidevertically below the electrode E1. The electrodes E1 and E2 are parallelto each other and to the path of the light beam B which lies midwaybetween them.

When a modulation signal voltage is applied by the source M1 to theelectrodes El and E2 an electric field is established in the crystal X1.The gradient of the electric field is a maximum in the plane between theelectrodes E1 and E2, which is parallel to the c-axis of the crystal andcuts through the center of the light beam B, and it decreases rapidly onboth sides of this plane. Due to the electro-optic nature andorientation of the crystal, this field variation produces acorresponding refractive index variation across any plane transverse tothe beam path. The refractive index will be most affected where thefield is greatest, that is in the center of the light beam B. Therefractive index variations resulting from the inhomogeneity of theelectric field will tend to spread the sides of the beam away from itscenter spreading mainly in the direction of the maximum electric fieldgradients. The light beam B therefore spreads over an area greater thanthe area of the optical aperture formed by the input end of thefiber-optic light guide G. The extent, of the beam spreading and theamount of light transmitted through the fixed aperture formed by the endof the light guide G will be determined by the instantaneous strength ofthe electric field and therefore by the modulation signal voltage. Hencethe light transmitted through the light guide G may be amplitudemodulated by variations of the modulation signal voltage.

Modulation voltages of either polarity can cause the beam to spread, andthe beam spreading is insensitive to the polarity of the appliedvoltage. Hence if the modulation voltage is an alternating voltage whichreverses its polarity, the light transmitted through the light guide Gwill tend to be amplitude-modulated according to a wave-form resemblingthe square of the modulation voltage and therefore having a substantialcomponent at twice the frequency of the modulation voltage signal. Thisfrequency-doubling effect can be avoided if the modulation signal sourceM1 is arranged to provide a desired modulation signal of suitableamplitude superimposed on a comparatively large direct bias voltage.

Obviously modulation can be achieved with other electrode arrangementsin other embodiments of the invention. For instance the electrode E2 maycover the whole of the underside of the crystal Xl. For high efficiencythe field should be greatest along the center of the path of the lightbeam. Another possible alternative arrangement is illustrated by FIG. 2.This drawing shows a transverse cross-section of an ADP crystal, havingtwo narrow linear longitudinal electrodes E1 and E4 parallel to eachother in one of its surfaces and two narrow linear longitudinalelectrodes E2 and E3 parallel to each other on the opposite surface. Thecrystal of FIG. 2 may be used in place of the crystal X1 of FIG. 1, withits crystal axis oriented as hereinbefore described for the crystal X1,and with the path of the light beam passing parallel to the electrodesE1, E2, E3 and E4, along the axis of symmetry of the arrangement ofelectrodes (at right angles to the plane of the paper in FIG. 2). Theelectrodes E1 and E3 are connected to one terminal of the modulationsignal source and the electrodes E2 and E4 are connected to the other.

As hereinbefore mentioned, the light guide G could be replaced by anoptical aperture in the form of a transparent pinhole in an opaquescreen.

The invention clearly requires only a simple arrangement with only onecrystal and one aperture to be aligned with the light beam. Since onlyone type of ray is used, and the production of a specified deflectionfor a specified signal is not essential, its operation is not criticallydependent on temperature and it is not likely to need any readjustmentto compensate for temperature changes. The optical transmissivity of thesystem can be comparatively high, especially when a pinhole aperture isused, for there need only be two interfaces (the ends of the crystal) tocause reflection losses.

The aperture need not necessarily be close to the modulator crystal, butmay be separated from it by any convenient distance. The output signalfrom the system may be used to align the aperature with the beam, and amodified form of the apparatus may be used as an optical alignmentdevice as will now be described. For alignment, a modulation signal ofknown frequency is applied to the crystal and the position of theaperture is adjusted to maximize the output signal at the secondharmonic of the modulation signal. If no direct-voltage bias issuperimposed on the modulation signal, the freq uency-doubling effectwill tend to be greatest when the optical aperture is centered about thepath of the beam. Control signals developed from the second harmoniccontent of the output of a simple system as shown in FIG. 1 will be mosteffective for guiding alignment in one dimension the dimension ofmaximum spreading of the beam. To get equally sensitive alignment in twoorthogonal directions at right angles to the path of the beam, twomodulator crystals may be used in tandem with modulation voltages ofdifferent frequencies applied to them in two orthogonal directions. Twocontrol signals can then be respectively derived from output signalcomponents at the second harmonics of the two modulation frequencies.Apparatus of this kind will now be described with reference to FIG. 3.

FIG. 3 shows a laser L arranged to project a beam B of coherent lightlongitudinally through two long slender ADP crystals X1 and X2 arrangedin tandem and on to one end of a fiber-optic light guide G. The guide Gis held in a holder H which is slideable on a pillar P upstanding from amoveable carriage 6. The

holder l-l may be moved horizontally by a servo motor J1 and a leadscrew S1 and vertically by a servo motor J2 and a lead screw S2. Themotor J2 is mounted on the carriage 6 and the motor J1 is mounted on arigid base (not shown). The end of the light guide G remote from thecrystals X1 and X2 is connected to an optical detector K. The detector Khas two outputs one of which is connected to the input of a filter andrectifier F 1 while the other is connected to the input of a filter andrectifier F2. The filter F1 selects a signal component of frequency 2f1,the filter F2 selects a signal component of frequency 2f2. The output ofthe filter and rectifier F1 is connected via an amplifier A1 to drivethe servo motor J l and the output of the filter and rectifier F2 isconnected via an amplifier A2 to drive the servo motor J2. A modulationsignal source M1 is electrically connected to the narrow linearelectrodes El and E2 of the crystal X1. A modulation signal source M2 iselectrically connected to the narrow linear electrodes E3 and E4 of thecrystal X2. The crystals X1 and X2 are similar but are arranged so thatthe modulation fields applied to them will be at right angles to eachother, that is to say X2 has been rotated anticlockwise with respect toX1, about the path of the beam. The modulation source Ml supplies analternating modulation signal of frequency fl while the modulationsource M2 supplies an alternating modulation signal of differentfrequency f2.

The mode of operation of the embodiment of FIG. 3 will now be described.As mentioned hereinbefore the frequency doubling effect is greatest whenthe beam is centered on the optical aperture formed in this case by theend of the light guide G, hence alignment is achieved when the secondharmonics of both modulating frequencies are at a maximum in themodulated light beam. In the embodiment of FIG. 3 the laser beam B isdirected through the crystals X1 and X2 on to the light guide G. Themodulations are impressed on the beam in the crystals X1 and X2substantially as hereinbefore described in relation to the apparatus ofFIG. 1. However in this case owing to the presence and the orientationof the crystal X2 the beam spreading is modulated in both the verticaland horizontal directions, and the illumination of the optical aperturewill be modulated at a frequency of twice f2 as well as at the frequencytwice fl. The detector K produces electrical signals which include thefrequencies 2fl and 2f2.

These signals are separated by the filters F1 and F 2 which also includerectifiers and smoothing circuits (not shown). The signals from thefilters F 1 and F2 are amplified by the amplifiers Al and A2respectively and are supplied to the servo systems J l and J2respectively. The servos J l and J2 drive the lead screws S1 and S2respectively and so alter the horizontal and vertical position of thecarriage 6 and consequently adjust the light guide holder H. The servosare arranged to stop driving the lead screws when the amplitude of thesecond harmonics of the frequencies fl and f2 are at a maximum. Thiscondition occurs when the light beam B is centered on the end of thelight guide G. A similar system may be used to center any opticalaperture about a light beam, the aperture being at any desired distancefrom the modulator crystals.

I claim:

1. An electro-optical modulator comprising a first electro-optic crystalwith first electrode means for applying a variable electric fieldthereto, a second electro-optic crystal with second electrode means forapplying a variable electric field thereto, the said first and secondcrystals having their c-axes orthogonal to one another, optical aperturemeans of predetermined cross-sectional area, means for projecting a beamof light through the first crystal and thencethrough the second crystaland onto the said optical aperture means, a first modulation signalsource means for applying a first alternating signal having a firstfrequency to the said first electrode means, a second modulation signalsource means for applying a second alternating signal having a secondfrequency to the said second electrode means, optical detector meansconnected to the said optical aperture means so as to receive light fromthe said beam of light through the said optical aperture means, servomeans connected to position the said optical aperture means in twoorthogonal directions, filter means for filtering the output of theoptical detector means so as to provide two output signals consequentupon the first and second alternating signals respectively, and meansfor applying the said two output signals to the servo means to centerthe said optical aperture means in the said light beam.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 1 DatedSeptember lnvenrofls) us John Dore It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE HEADING:

I Please add the following:

Foreign Application Priority Data August 15, 1968 Great Britain. .No.38997/68 Signed and sealed this 13th day of March 1973.

(SEAL) Attest:

M.FLETCHER JR. ROBERT 5 OTTSCHALK fi g g s ing Officer Commissloner ofPatents FORM PC4050 (10-69) UscoMM-Oc 00 75 1 n U 54 GOVERNMN' PRNYINGOTFICE "9 0-366!!! UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION3,691,u8 4 Dated September 12, 1972 Patent No.

Inventor(s) Marcus John Dore It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE HEADING:

' Please add the following:

Foreign Application Priority Data August 15, 1968 Great Britain. .No.38997/68 Signed and sealed this 13th day of March 1973.

(SEAL) Attest:

.PLETCHER,JR. ROBERT GOTTSCHALK EDWARD M Commissioner of PatentsAttesting Officer USCOMM-DC 603764 69 u 5 GOVERNMINT rnmrmc. orruczn69o3u-u4 FORM PO-IOSO (10-69)

1. An electro-optical modulator comprising a first electro-optic crystalwith first electrode means for applying a variable electric fieldthereto, a second electro-optic crystal with second electrode means forapplying a variable electric field thereto, the said first and secondcrystals having their c-axes orthogonal to one another, optical aperturemeans of predetermined cross-sectional area, means for projecting a beamof light through the first crystal and thence through the second crystaland onto the said optical aperture means, a first modulation signalsource means for applying a first alternating signal having a firstfrequency to the said first electrode means, a second modulation signalsource means for applying a second alternating signal having a secondfrequency to the said second electrode means, optical detector meansconnected to the said optical aperture means so as to receive light fromthe said beam of light through the said optical aperture means, servomeans connected to position the said optical aperture means in twoorthogonal directions, filter means for filtering the output of theoptical detector means so as to provide two output signals consequentupon the first and second alternating signals respectively, and meansfor applying the said two output signals to the servo means to centerthe said optical aperture means in the said light beam.